Immune-Mediated Thrombocytopenia In Pets

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook

Thrombocytopenia means an abnormally low platelet count (also known as thrombocytes). Platelets are tiny cells in the blood which stick together to plug sources of bleeding. Having too few platelets will prevent the blood from being able to clot and, if numbers drop too low, can lead to spontaneous bleeding. The most common cause of low platelets is Immune-Mediated Thrombocytopenia, commonly referred to as IMTP, in which a dysfunction of the immune system leads to the body attacking and destroying its own platelets. This is a potentially serious condition seen mostly in dogs but can also occur in cats. 

IMTP can be primary or secondary. In primary IMTP, there is no underlying cause. In secondary IMTP, the platelet destruction occurs due to an underlying condition, such as a cancer, infection, or reaction to a drug. IMTP is seen more commonly in middle-aged female dogs, and in certain breeds, including Cocker Spaniels, Old English Sheepdogs, Golden Retrievers and Poodles, though it can affect any age or breed, including crossbreeds. Secondary IMTP that is caused by an infection is more common in pets who have travelled outside of the UK. 

Sometimes, IMTP is seen in combination with Immune-Mediated Haemolytic Anaemia (IMHA). This is when red blood cells are also destroyed. Both conditions together can be referred to as Evan’s syndrome. 

What are the symptoms of Immune-Mediated Thrombocytopenia? 

IMTP often develops slowly, with no visible symptoms until platelet numbers reach a critical level. Occasionally, IMTP is identified due to a routine blood test (such as before surgery), rather than because the pet is unwell.   

The symptoms of IMTP are caused by bleeding due to the low platelet numbers. Affected animals will often have signs of bruising, especially on the gums and the thin skin around the groin and abdomen. They may also have blood in their vomit, stools or urine, experience nosebleeds (known as epistaxis), or have bleeding that doesn’t stop after minor trauma. Rarely, bleeding can occur internally, such as into the chest cavity, abdomen, or brain. Visible bruising is less common in cats, where IMTP is more likely to be identified only on a blood test. Patients with secondary IMTP may have other symptoms caused by an underlying condition. 

Which tests are used to diagnose Immune-Mediated Thrombocytopenia? 

The first step in diagnosis of IMTP is a general blood test. This will include a platelet count and check for other abnormalities, such as anaemia. Automated analysers can give unreliable platelet counts as the cells are so small, and often stick together. If in doubt, a blood smear is made and the platelets counted manually under a high power microscope. This may be done in-house, or sent to an external lab for review by a specialist pathologist.   

There is no specific test for IMTP, so a diagnosis is made when we are confident that other causes of Thrombocytopenia (low platelet count) or bleeding have been ruled out. Secondary causes of IMTP are commonly screened for using diagnostic imaging, such as x-ray and/or ultrasound examination, and further blood tests. Platelets are made in the bone marrow, so occasionally a bone marrow biopsy is required to be certain that enough platelets are being produced. 

How is Immune-Mediated Thrombocytopenia treated?

Treatment of primary IMTP predominantly involves suppression of the immune system to halt platelet destruction. This is typically achieved using glucocorticoids (steroids) but depending on the severity of disease, other drugs might be used in combination. Often, these are human immunosuppressant drugs and are not specifically licensed for use in animals. Additional medications may be given to help manage symptoms, such as anti-nausea drugs, gastro-protectants, and antibiotics if infection is present.

Depending on the severity of disease, patients with IMTP may be managed at home, or may need to be hospitalised. Patients who are bright, eating and drinking are able to take medication at home, but will need to return for regular blood tests. Patients who are lethargic, not eating or severely thrombocytopenic may require hospitalisation for intravenous medication and close monitoring. All pets with Thrombocytopenia must be handled very carefully to prevent further bruising. They should be rested, and not allowed to run around or play until platelet numbers are high enough to avoid spontaneous bleeding. Blood tests are required to monitor platelet count. However, repeat sampling can cause more bleeding, so it is important to perform only as many tests as strictly necessary – your vet will guide you with this. 

Both red blood cells and platelets can be administered via a blood transfusion. However, there is a small risk of allergic reaction, and a possibility that the immune system will destroy the new cells. Transfusion may be recommended if there is concurrent anaemia (due to blood loss or IMHA), or bleeding is severe and a short-term increase in platelet numbers likely to be beneficial. However, for most patients, a blood transfusion is not a good method to improve Thrombocytopenia. 

Response to treatment is measured by looking at the platelet numbers in a fresh blood sample. Platelets can be difficult to count, especially if there are not many of them, so it is common to send samples to a specialist lab. A normal platelet count for a dog is over 200 x 109/L, with a count of less than 50 x 109/L considered severely thrombocytopenic. Patients who respond well to treatment can reach normal platelet levels as quickly as 3-7 days after starting treatment, though this is not always the case. 

What is the outlook for pets with Immune-Mediated Thrombocytopenia? 

IMTP is a serious disease, and not all pets will respond to treatment. It is estimated that over 70% of dogs respond well. However, the remaining cases may not survive or require euthanasia due to lack of response to treatment or complications of the disease, such as bleeding. There is less information about IMTP in cats, but survival rates are similar to dogs. 

Patients who respond to treatment will need to take immuno-suppressants long term. Once they are stable, treatment can be very gradually withdrawn. Often, one medication is stopped at a time, and typically the dose is reduced by 25% every few weeks – although this is case dependant and will be tailored to each pet. Platelet count is monitored closely throughout. Some patients will tolerate this well and eventually cope without medication, while others may need some degree of treatment for life. 

Once a pet has had IMTP, they are at risk for relapse months or years later. There is no need to continue treatment if platelet count is stable, however, monitoring every 6-12 months is wise and any clinical signs should be promptly investigated. 


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Page last reviewed: 23rd April 2024

Next review due: 23rd April 2026